1. Field of the Invention
The present invention relates to instrumentation for measuring and controlling the flow of fluids, and more particularly to an assembly for measuring the mass flow of gases which can be incorporated into a flow controller or flowmeter.
2. Description of the Related Art
The measurement and control of the mass flow of gases is important in many industries. During the manufacture of semiconductor chips, for example, many of the processes require precise reaction of two or more gases under carefully controlled conditions. Since chemical reactions occur at the molecular level, control of mass flow is the most direct way to regulate the absolute and relative quantities of gaseous reactants.
There have been developed in the art a variety of instruments for measuring the mass flow rate of gases from less than one standard cubic centimeter(s) per minute (sccm) to more than 500,000 sccm. The prevalent design of such instruments requires that the flowing gas be divided into at least two portions. In a typical instrument, a relatively small portion of the total flow is routed through a sensor assembly where the mass flow is measured, while the remainder, i.e., almost all of the flow, is routed through a flow splitter assembly disposed generally parallel to the sensor assembly. The sensor assembly includes a capillary tube around which are wound two resistance thermometers as identical as possible in electrical and mechanical characteristics. Each thermometer is wound in a tight coil in thermal communication with the outer surface of the tube. The thermometers form two legs of an electronic bridge; the other two legs are usually fixed resistors. When a voltage is applied across the bridge, current flows through each thermometer, causing it to self-heat. When there is no flow of gas through the capillary tube, the thermometers heat up identically. As gas begins to flow through the tube, the gas first cools the upstream thermometer and then the downstream thermometer which is cooled less because the gas is now slightly warmer due to heating by the upstream thermometer. The resultant temperature differential is a function of both the mass flow rate and the properties of the particular gas. As disclosed in U.S. Pat. No. 1,193,488, C. C. Thomas more than eighty years ago utilized separate sensor and flow splitter assemblies and resistance thermometers in designing gas flowmeters. In U.S. Pat. No. 1,222,492, he disclosed a flowmeter which measured the rate of gas mass flow by imparting heat to raise the gas temperature while automatically regulating the imparting of heat so as to keep constant either the rate at which heat is imparted or the temperature rise produced.
Typically, a signal conditioning circuit is used to compensate for nonlinearities in sensor response, account for changes in operating parameters such as resistance values, and convert analog output signals into digital format. Variations of the basic sensor assembly design include: interposing a heater coil between the two thermometer coils and reducing the bridge current so that the thermometers do not self-heat; substituting thermocouples or thermistors for the resistance thermometers; or arranging the signal conditioning circuit so that it maintains each thermometer at the same temperature, using the difference in wattage to the coils to determine the mass flow rate.
In some semiconductor processes such as ion implantation, the process gases must be stored and handled with great care and not be wasted, because they are toxic, highly reactive and expensive. Examples are arsine, phosphine and boron trifluoride. An ion implanter chamber operates at a very low pressure approaching a hard vacuum, drawing small portions of gas at low flow rates, typically 0.25 to 10 sccm, out of a storage container through a mass flow controller. Should an accident occur breaking the vacuum, gas stored conventionally in a pressurized container would be released into the environment, with expensive if not hazardous consequences. As disclosed in U.S. Pat. Nos. 5,518,528, 5,704,965 and 5,707,424, a gas storage system has been developed by Advanced Technology Materials, Inc. (ATMI) of Danbury, Conn. wherein gases are stored at slightly less than atmospheric pressure in special containers filled with porous resin beads which adsorb gas molecules on their collectively large total surface area. In the event of a vacuum break there will be minimal escape of gas. The amount of gas in a container varies non-linearly with pressure; the bottle is 100% full at atmospheric pressure (760 Torr), but may still be half full at 75 Torr. Since the gas is expensive and since it is also expensive to shut down and change the container, it is desirable to be able to operate the system at the lowest possible container pressure to avoid wasting gas and having frequent shutdowns. In order to withdraw essentially all of the stored gas, it is important that the mass flow controller have a very low pressure drop at the rated flow of the implanter. With existing controllers, the combined pressure drop through the sensor and flow splitter assemblies at a nominal flow rate of 5 sccm and an exit pressure of zero Torr (common operating parameters) results in a residual gas pressure in the container of 50 to 80 Torr. Because of the mechanism whereby the gas desorbs from the porous beads, at such pressures only about 60 percent of the gas can be extracted, resulting in a substantial economic penalty. Pressure drop in the flow controller is due to the small diameter of the sensor tubing, resistance of the flow splitter, multiple right angle turns traversed by gas entering and exiting the sensor assembly, and losses at the entrance and exit of the sensor assembly. Thus, there is a need for a mass flow controller which has sufficiently low pressure drop that most of the gas can be extracted. At a flow rate of 5 sccm, this requires that the mass flow controller be operative at container pressures of 10 Torr and below.
Semiconductor fabrication equipment such as ion implanters require that a ass flow controller have a response time of less than 4 to 5 seconds to avoid safety interrupts. To meet this requirement, the capillary tube should be as small as possible to minimize thermal mass. This is the principal reason why the prevalent design of such instrumentations divides the flow into two paths. Capillary tubes presently in use, which have a bore about 0.010 inch in diameter and a wall thickness of about 0.002 inch, have a response time of about 1 second. However, a bore this narrow is unsuitable for an in-line mass flow controller where all the gas goes through the sensor assembly. Thus, there is a need for a mass flow controller with a flow measuring assembly which allows in-line flow rates in the 0.25 to 10 sccm regime, yet has a suitably short time constant.
U.S. Pat. No. 4,984,460 ("'460") to Y. Isoda, entitled "Mass Flowmeter" is directed to a mass flowmeter adapted for low fluid flow rates, typically about 5 sccm. The flowmeter has a single conduit, rather than separate flow splitter and sensor assemblies, within which are mounted an upstream and a downstream resistance thermometer coil, each included in a separate constant temperature difference circuit. Each circuit also includes an ambient temperature detecting resistor having approximately the same resistance as the coil and a temperature difference setting resistor, connected in series to the coil. The circuits are connected to a control unit which controls the differences between the temperatures of the coils and the ambient temperature to be approximately the same as a value set by the temperature difference setting resistor. Mass flow rate is measured by detecting a difference in the amounts of energy supplied to the coils.
U.S. Pat. No. 4,487,062 ("'062") J. G. Olin et al., entitled "Mass Flowmeter" is directed to a flowmeter for measuring the mass flow and flow rate of gasoline, oil, cooling liquids or other fluids. The flowmeter includes a straight sensor assembly disposed parallel to a flow splitter assembly, and is configured so that fluid entering and exiting the sensor assembly must traverse a sequence of four right-angle turns. The sensor assembly includes a capillary tube around which are wound two resistance thermometer coils which are segments of a single coil with an electrical lead connected at its center. An evacuated enclosure surrounding the coils prevents convection effects between them and conductive transfer of heat away from the capillary tube, and also acts to reduce coil response time.
U.S. Pat. No. 3,650,151 ("'151") to C. F. Drexel, entitled "Fluid Flow Measuring System," is directed to a control system which permits the absolute mass flow rate of a fluid to be monitored and controlled regardless of changes in state parameters such as pressures and temperatures. The system is adapted for use in chemical processes where a liquid is vaporized by passing a carrier gas through or over it so that the carrier gas transports the vaporized liquid into a reaction chamber. The system also is adaptable to mixing gases, and controlling the amount of a sublimating solid by varying the flow rate of carrier gas through a bed of the solid. The system includes a mass flow sensor assembly in parallel with a conduit tube. The sensor assembly includes a capillary tube around which are wound a heater element disposed between two resistance thermometer coils. The coils are connected with two fixed resistors in a bridge circuit which is part of a signal conditioner that provides a linear output voltage as a function of mass flow.
U.S. Pat. No. 2,729,976 ("'976") to J. H. Laub, entitled "Thermal Flowmeter" discloses a flowmeter having a conduit through which flows a liquid or gas. The conduit includes a metallic pipe section thermally insulated from the adjacent upstream and downstream conduit sections. Heat is transferred to the flowing medium by means of a heater coil wound around the pipe section which is upstream of a resistance thermometer coil. Second and third coils wound around the upstream conduit section serve as a reference resistance thermometer and a temperature compensator. Three additional embodiments each utilize a sensor assembly parallel to the conduit. In one embodiment the pressure drop across the sensor assembly is "relatively small." Fluid entering and exiting the sensor assembly must traverse a sequence of right-angle and arcuate turns.
None of these references discloses a sensor assembly which achieves both low pressure drop and fast response time. The '460 and '151 patents address neither roblem. The '062 patent purports to provide a fast response time, but no data are shown. In one embodiment the '976 patent provides a "relatively small" pressure drop, but again no data are shown.
Increasing the cross-section of a conduit in the direction of flow of a fluid, either suddenly or gradually, reduces the velocity of flow and converts a portion of the kinetic energy of flow into pressure energy. It has long been known that by increasing the cross-section gradually, shock and consequent eddy formation and loss of energy would be reduced. Too rapid an expansion rate, so that the fluid rapidly decelerates, causes localized back-flow resulting in kinetic losses; limiting the expansion rate reduces back-flow. The phenomenon of back-flow of a compressible or incompressible fluid at the wall of a divergent circular tube was addressed by H. Blasius in "Laminare Stromung in Kanalen wechseldner Breite," Zei tschrift f. Mathematik U. Physik, 59, 225 (1910). Blasius found that the condition for avoiding back-flow in an expansion region is for the differential rate of increase of conduit cross-section with respect to the differential change in axial location along the fluid path to be a numerical constant divided by the fluid's Reynolds number. This condition requires that the angle at which the tube diverges be small.
Blasius's prediction was confirmed by A. H. Gibson in experiments described in "The Conversion of Kinetic to Pressure Energy in the Flow of Water Through Passages Having Divergent Boundaries," Engineering, vol. 93, 205 (1912). Gibson found that for a circular pipe with boundaries uniformly diverging at an angle .theta., the minimum loss of pressure energy occurs when .theta. is about 6.degree. . The loss is made up of two parts due, respectively, to wall friction and to shock following cross-section enlargement. As .theta. is reduced, the length of pipe, and therefore the friction loss, is increased; for values of .theta. less than 6.degree., the increased friction loss more than counterbalances the reduced shock loss. As .theta. is increased, the loss rapidly increases, attaining a maximum in the neighborhood of 65.degree.. At large divergence angles, a gradual enlargement of the cross-section results in a greater pressure loss than a sudden enlargement.